1. Field Of The Invention
The field of the invention is electric heater assemblies, and more specifically, a thermostat control circuit for an electric heater assembly.
2. Description Of The Prior Art
Heretofore it has been proposed to control the current to a heating element load in response to the temperature of the heating element. An example of such a temperature control system is disclosed in U.S. Pat. No. 3,339,942.
Also various circuits for interrupting current to an electric heating unit when a preselected maximum temperature has been reached or when some malfunction in the heating unit occurs, have been proposed. In this respect reference is made to the various circuits disclosed in U.S. Pat. Nos.: 3,760,157, 3,794,809, 3,832,527, 3,848,111.
Furthermore it has been proposed to operate circuit breakers in response to an increase in leakage current from a load. Such circuits operate on the basis of an imbalance in the current to the load and the current from the load in the supply lines to the load. Examples of such leakage current operated circuit interrupters are disclosed in U.S. Pat. Nos.: 3,654,515, 3,789,268, 3,813,579, 3,800,121.
The load in U.S. Pat. No. 3,800,121, referred to above is a three-phase electric heating apparatus, and this patent discloses a circuit for controlling current to the heating apparatus in response to leakage current from the three-phase load.
Still further, it has also been proposed to utilize the change in resistance due to the change in temperature in a semi-conducting material, such as ceramic material, to measure the temperature of the ceramic material or an element juxtaposed thereto. In this respect, U.S. Pat. No. 3,679,473 suggests that the variable resistance of a ceramic material can be used to determine the temperature of a body, either separately or in joint function with its use as a heating element.
Other prior art patents which utilize leakage current through, or the change in resistance of, an insulating material or semi-conductive material are disclosed in the following U.S. Pat. Nos.: 2,941,192, 2,745,284, 3,483,750.
As will be described in greater detail hereafter, the present invention differs from the prior art apparatus and circuits referred to above in that the heater assembly of the present invention and the leakage current thermostat utilized therein include a piece of ceramic material in contact with a heating element and control circuitry responsive to changes in the leakage current through the ceramic material to control the current to the heating element and thereby the temperature of the heating element.